The present invention broadly relates to continuous control or regulation devices and, more specifically, pertains to a new and improved construction of an electro-hydraulic actuator or positioning drive for performing continuous control or regulation operations.
Generally speaking, the positioning drive or actuator of the present invention comprises a vibrating armature pump for delivering a hydraulic fluid, a pressure-operated return valve, an actuation member and a return spring acting upon the actuation member. The actuation member is displaceable by the hydraulic fluid against the force of the return spring in dependence of an externally supplied positioning or adjustment signal.
A considerable disadvantage of known actuators or positioning drives of this type is the requisite supply of an operating voltage for the vibrating armature pump and of an adjustment or positioning signal for controlling the actuation member which are mutually independent of one another. The actuation member may be a piston or a membrane. The actuation member is controlled either by on-off control of the pump or by influencing the pressure generated by the pump. In either case, two non-interchangeable pairs of electrical conductors are required for operating the actuator or positioning drive.
Since the actuation member operates against the force of a return spring, a pressure-dependent return valve having a throttling function is required in order to permit return flow of the hydraulic fluid delivered by the pump.
An actuator or positioning device known from the French Patent Publication No. 2,324,914, published Apr. 15, 1977, comprises an electrically controlled solenoid valve as the return valve which requires yet a further conductor.
On the other hand, the French Pat. No. 1,311,466, granted Oct. 29, 1962, discloses an actuator or positioning drive having a self-operating return valve in the form of a valve combination. This valve combination comprises a small piston-operated valve which closes against the force of a return spring under a pressure increase of the pressure medium delivered by the pump and a spring-loaded pressure retention valve which permits access to the actuation member under an increase in pressure. This known actuator or positioning drive is therefore not only complicated in construction but also displays appreciable leakage and pressure losses.
A piston serving as an actuation member is generally sealed against the cylinder by means of O-ring or stuffing box seals. The friction forces arising in this type of seal produce a hysteresis effect in actuators or positioning drives operating against the force of a spring and which hysteresis effect is so great that it is hardly possible to employ such actuators or positioning drives for continuous control or regulation operations.